Reported in the present article are the results of investigations on the biosolubilization of a low-rank coal, viz. lignite (from Neyveli, Tamil Nadu, India) into humic acid. The solubilization of lignite was effected at ambient temperature and pressure with the aid of some potent fungal strains isolated from soil samples collected from Neyveli lignite mines. The isolated fungal microbes were characterized and studied under controlled conditions for optimum solubilization of lignite. Of the isolated fungal strains, Aspergillus fumigatus showed maximum (22.3%, w/w) solubilization, as contrasted with other isolated microbes, e. g., Fusarium udum, Fusarium solani, Aspergillus oryzae, and Aspergillus sydowii, which showed much less solubilization, being in the range of 4.5-16.8% (w/w). The process parameters were optimized in respect of media, initial pH, incubation period, and substrate concentration for maximum yield of the product. The solubilization becoming constant after a certain dose of lignite (10 g/100 ml) has been explained on the basis of non-availability of the surface area on the lignite particles owing to clogging/blockage of pores by the debris of dead cells and their metabolites. It could also be attributed to the broth becoming too dense, thereby inhibiting the growth and propagation of the microbial cells or possibly due to inhibition by the substrate at higher doses.